Quantitative effects of sympathetic and vagal nerve stimulations on sinus and AV junctional rhythms.

Responses of the sinus node and atrioventricular (AV) junctional pacemakers to autonomic denervation and to individual stimulations of the right and left stellate and both vagi were studied in 33 anesthetized dogs. Autonomic denervation depressed sinus node automaticity by only 18% from control, whilst AV junctional automaticity was reduced by 48.5% from control. Sympathetic and parasympathetic stimulation frequency-response curves (0.25, 0.5, 1, 2, 4, 8, 16 and 32 Hz) were obtained. In the sinus node the chronotropic responses to sympathetic stimulations reflect a bilaterally asymmetrical innervation with a right sided preponderance. In contrast, sinus slowing in response to either right or left vagal stimulations were indistinguishable when lower frequencies of stimulation were used. At 4 Hz and higher frequencies there is a right vagal preponderance. The AV junctional chronotropic responses suggest that this major subsidiary pacemaker receives a bilaterally symmetrical autonomic innervation. The chronotropic responses to individual nerve stimulations expressed as percent changes in sinus rate and AV junctional rate from their respective controls after autonomic denervation show that the AV junction is far more responsive than the sinus node to both sympathetic and parasympathetic stimulations. To allow for more meaningful comparisons the data were normalized using the respective maximum increase and maximum decrease of sinus node and AV junctional rates to left and right sympathetic and parasympathetic stimulations as the 100% reference. These normalized curves show that 50% of the maximal chronotropic responses were always achieved at a lower stimulus frequency in the AV junction than in the sinus node; shift of the AV junctional response curves to the left of the sinus node response curves by a 0.2 (sympathetic) and 0.3 (parasympathetic) log units was observed. These studies further showed that sympathetic activity in the AV junction is an absolute prerequisite to maintain regular AV junctional rhythms especially during the bradycardic episodes evoked in the study of vagal stimulus frequency-response curves.